In recent years, a problem has arisen of reducing CO2 gas, which is said to be a cause of global warming, or the exhaustion of oil or other fossil fuels in the future. To address these problems, recyclable natural energy has been actively used. Wind power is one form of recyclable natural energy, and large-scale wind power has been increasingly used on a worldwide level.
The most suitable area for wind power generation is an area where strong wind is expected to blow constantly, and off-shore wind power generators are under planning or actually in operation all over the world (see Patent Documents 1 to 4). In order to build a tower for wind power generation at sea, it is necessary to drive the foundation portion of the tower into the sea bed. Further, in order to obtain sufficient height of the turbine blade of the wind power generator from the sea level, the foundation portion of the tower is required to have sufficient length, rigidity, and strength.
Thus, the foundation portion of the tower of the wind power generator employs a pipe structure having a wall thickness exceeding 50 mm, for example, of approximately 100 mm, and a large diameter in cross-section of approximately 4 m. Further, the total height of the tower is as high as 80 m or more. Such a large structure is required to be welded and built on the coast near the construction site in an easy and efficient manner.
Under the circumstances described above, there arises a new demand for welding an ultra-thick steel plate having a thickness of 100 mm on-site in a highly efficient manner.
In general, efficiency in welding can be increased by employing a high-energy density beam such as an electron beam and laser beam. However, welding through the electron beam is required to be performed in a vacuum chamber under a high vacuum state, for example, of 0.1 Pa or less. Thus, the thickness of the steel plate to be welded has been limited.
In recent years, to address the circumstances described above, the Welding Institute of the United Kingdom has proposed and developed a welding method (reduced pressure electron beam welding: RPEBW) enabling working under a low vacuum state, for example, of 10 Pa or less, as a welding method enabling efficient welding of an ultra-thick steel plate with a thickness of approximately 100 mm on-site (Patent Document 5).
The tower of a wind power generator at sea is constantly exposed to vibration due to strong wind as described above, and the structure body of the foundation portion and the steel pipe pillar constantly receives repeated loads. Thus, the welded portions are required to have fatigue resistance against vibration under the giga-cycle range, the order of which is higher than ordinary fatigue cycles. In particular, stress increasingly concentrates on the weld toe of the weld bead, causing a reduction in the fatigue strength against repeated loads.
As a measure for alleviating the stress concentration on the weld toe of the weld bead as described above, there has been proposed a technique of alleviating the stress concentration by increasing the radius of curvature of the weld bead 32, and a contact angle θ between a steel plate 31 and a weld bead 32 as illustrated in FIG. 5.
For example, Patent Document 6 proposes adjusting components of flux or components of shield gas to increase the radius of curvature and the contact angle θ. However, the method proposed by Patent Document 6 employs a gas shield arc welding, and is not applicable to welding through a high-energy density beam without using a shield gas.
Further, Patent Document 7 proposes setting the ratio of the thickness of the steel plate relative to the height of the weld bead to 0.2 or less to reduce stress concentration on the weld toe of the weld bead. However, this document only specifies the shape of the weld bead, and does not disclose the specific method for forming the above-described width of the weld bead or specific welding conditions for the forming of the weld bead. Thus, Patent Document 7 lacks reproducibility, and is difficult to be used industrially.